Hard disk drives have developed as an efficient and cost effective solution for data storage. Since the introduction of the first magnetic disk drive, storage density capabilities have increased by eight fold, with an average steady increase of nearly fifty percent per year. Main stream technology has consisted of storing information on continuous granular media having out-of-plane anisotropy and being associated with a soft under layer which helps concentrate the magnetic flux underneath the write pole of the head, thus increasing the write field efficiency.
However, it is generally accepted that this technology will reach its limit at an areal density between 500 Gbit/in2 and 1 Terabit/in2. This limit is set by the so-called “recording trilemma” which is the difficulty to reconcile three requirements of magnetic recording technology: i) a sufficient number of grains per bit to insure a large enough signal to noise ratio, ii) a sufficient stability of the magnetization of each grain against thermal fluctuations, iii) the ability to switch the magnetization of the grain with the field available from the write head. Several solutions are under investigation to circumvent this trilemma, including Heat Assisted Magnetic Recording (HAMR), Microwave Assisted Magnetic Recording (MAMR), bit-patterned media, with combinations of these approaches also being possible.
Bit-patterned media, in particular, presents one of the most promising methods to overcome the density limitations imposed by the trilemma. In conventional media, the magnetic recording layer is a thin film of a magnetic alloy, which naturally forms a random mosaic of nanometer-scale grains that behave as independent magnetic elements. Each recorded bit is made up of many of these random grains. In bit-patterned media, on the other hand, the magnetic layer is created as an ordered array of highly uniform islands or dots, each dot being capable of storing an individual bit. FIG. 1 depicts an exemplary recording disk 104 comprising an array of magnetic dots 108. Each magnetic dot 108 is capable of storing a single bit of information.
One challenge associated with bit-patterned media is that it has been shown that magnetic moments at the edge of a magnetic dot 108 cause the dot 108 to become unstable at its boundaries, thereby decreasing the signal-to-noise ratio at the edge of the magnetic dot 108. The likelihood of disk malfunctions (e.g., read or write errors) increases as the signal-to-noise ratio of the dot decreases. In addition to affecting the signal-to-noise ratio and stability of stored data, the defect or the fluctuation at the dot 108 edge is also one of the primary sources of media switching field distributions. Therefore, with these convention bit-patterned media designs, the achievable areal density is sacrificed significantly.